This invention relates to a method and apparatus for domestic processing of food and more particularly it relates to such food processing by exchange of heat between latent heat storing materials and the food by forced circulation of a liquid thermal exchange fluid.
Although thermal exchange fluid systems are known in various industrial and commercial applications, such prior art does not include hot and cold reservoirs having latent heat storing materials which are used to advantage in the system of the invention. Liquid thermal exchange fluid systems generally comprise a thermal source, a user, and a circuit to circulate the thermal exchange fluid between them. Various prior systems indicate components and configurations which may be adapted to satisfy particular applications.
A representative industrial heating system for such applications as processing materials in chemical reactors or kettles circulates a stable organic liquid between a furnace and a jacket surrounding the processing vessel. The liquid circuit includes a pump to provide a more rapid flow and effective heat transfer across heat exchange surfaces and includes a regulator valve to control temperature of thermal exchange fluid at the jacket of the processing vessel. Some industrial processes, such as molding of thermoplastic materials, require alternate heating and cooling of molding dies. A cold liquid circuit, similar to the hot liquid circuit and also comprising a pump, conduits, and a heat exchanger in thermal contact with a cooling means such as cold water, delivers cool thermal exchange fluid to a selector valve which selects flow from the hot or the cold circuit for delivery through the molding die. With relatively short duration of the heating and cooling phases and continuous operation of the process, thermal storage by a sensible mode in the thermal exchange fluid is adequate and thermal capacity of heaters or other sources is approximately that of current user requirements. Although some domestic processes could benefit from the more precise temperature control and alternate heating and cooling capability provided by some of the industrial thermal exchange fluid systems, their generally intermittent use, wide range of heating and cooling demand, and a requirement of moderate initial and operating cost with performance significantly improved over conventional domestic heating and cooling means precludes use of the kinds of thermal exchange fluid systems which could be suggested by industrial thermal exchange fluid systems.
Latent heat storing systems with fluid heat transfer are used for domestic space or water heating where they provide charging of the storing system at more convenient times or under more economical conditions. A typical space heating system has a single liquid circuit which can be connected to hot and cold sources through a selector valve. Such systems do not provide the combination of hot and cold reservoirs with latent heat storage and a hot and a cold liquid circuit each connectable to a user.
Commercial systems based on thermal exchange fluids for processing foods at alternate high and low temperatures typically use either a vapor phase fluid, such as steam, for heating and its liquid phase for cooling or they use two different substances, such as steam and a refrigerant, in separate heat exchangers for heating and cooling. Separate fluids or heat exchangers would be undersirably complex for domestic appliance systems.
A domestic appliance system disclosed by A. N. A. Axlander in U.S. Pat. No. 3,543,001 uses a liquid thermal exchange fluid to accumulate heat in a hot reservoir and to transfer the heat by natural convection through appliance units such as ovens or stove plates. Although the system provides precise temperature control and charging of the hot reservoir at off-peak hours, it has disadvantages of heat storage in a sensible rather than latent mode which results in large temperature variation and size of the hot reservoir, natural rather than forced convection which limits heat transfer rate to the appliances, separate mains for each appliance which results in a complex conduit assembly, and lack of cooling capability which limits general use for a domestic appliance system.
Another domestic appliance system based on liquid thermal exchange fluid disclosed in my U.S. Pat. No. 3,888,303 has a thermal exchange fluid circulating between hot or cold reservoirs and connectable household units. Although forced circulation and heating and cooling capability are provided, the system lacks thermal storage in a latent mode, and a separate liquid circuit is used to deliver the thermal exchange fluid to each houseware unit. A general disadvantage is that special houseware units are required.
Conventional domestic appliances such as refrigerators, ovens, and ranges which are used for storage or processing of food are generally self-contained units and are not part of a system for storing and distributing thermal energy to optimize energy inputs, processing conditions, disposition of unwanted heat, and distribution of cost between assemblies for converting energy and for delivering the energy to food. Deficiencies of conventional appliances will become more apparent as they are compared to the system and method of the invention and include:
Transfer of waste heat into the kitchen where it may have to be removed by an air conditioner with further energy expenditure. Sources of such waste heat include residual appliance heat remaining after cooking has been completed and refrigerant condensation heat.
Since conventional appliances are self-contained, marginal cost of additional units does not decline. Accordingly, the number of specialized appliances which could increase the range of food processing environments is limited by cost. Further, combining heating and cooling functions in a single appliance is expensive by conventional means and would not be feasible for a large number of appliances. Yet combined heating and cooling capability has advantages for improving food values and for increasing conveniences such as unattended cooking. Proposed conventional conveyor means for transferring food from a refrigerator to an oven are excessively complex and limited.
Since energy is not stored in conventional appliances, inputs are scaled to present demand which results in costly power supply apparatus and use of expensive power during peak hours. Further, heat content and transfer in conventional ovens or ranges at high temperature and low velocity convection are seldom optimal. Substances of low viscosity, such as water for soups, coffee, or other processing, typically are not heated rapidly. Substances of high viscosity or solids, however, may be scorched at high temperature. A system capable of providing larger heat capacity and controlled temperatures would be desireable.